Servo actuators are used in a wide variety of industries to produce a controlled movement in response to an input signal. Servo actuators are typically used in industries where precise control of the output motion is required. For example, in the aerospace industry, servo actuators may be used to move the control surfaces of an aircraft.
A servo actuator includes a servo valve and an actuator. The servo valve alters the flow of a fluid through the servo actuator in order to control the position, velocity, acceleration or force generated by the actuator. Typical actuators include hydraulic or pneumatic cylinders or motors.
A servo valve typically comprises a moving element (spool) and a fixed element (sleeve). The relative movement of these two elements controls the flow of fluid through the valve in response to a mechanical or electrical input signal.
Additive manufacture, also known as 3D printing, is a term applied to processes whereby three-dimensional articles are manufactured by building up successive layers of material in different shapes. This is in contrast to traditional manufacturing techniques (known as subtractive manufacturing) such as milling or boring in which material is removed in order to create the final form of an article. The flexibility offered by additive manufacturing techniques allows the design of servo actuators to be approached differently. Redesigning a servo actuator taking into account the possibilities offered by flexible manufacturing has resulted in an improved design which overcomes a number of longstanding issues associated with servo actuators.
Servo actuators that are used in safety-critical applications, for example aerospace applications, must meet stringent safety criteria.
One mode of servo actuator failure is a “supply-failure” wherein the supply of pressurised fluid to or within the actuator is interrupted. Generally, in safety-critical applications servo actuators must continue to function in the case of a supply-failure. One way in which this can be achieved is to introduce redundancy into the control of the servo actuator. As part of this solution a second, independent, pressurised supply is provided. The second pressurised supply is connected to the actuator via a completely separate hydraulic system (i.e. a separate servo valve and flow galleries) in order to reduce the risk that failure of the first pressurised supply prevents the servo actuator from functioning. Having two hydraulic systems to control the same actuator may increase the size and weight of the servo actuator assembly.
Another way in which a servo actuator may fail is if the spool jams, for example as a result of contaminants in the fluid or wear of internal components. This mode of failure may be referred to as a “spool-jam”. In many safety critical applications the design of the servo actuator must ensure that pressurised fluid can be redirected in the case of a spool-jam. Such an escape mechanism is required to avoid a build-up of high pressure fluid within the servo actuator which may cause the actuator to run out of control or jam in position. If the pressurised fluid of the first hydraulic system can escape to return following a spool-jam, the second hydraulic system may continue controlling the actuator. Typically, this is achieved by providing an independently operated bypass valve at a point in the hydraulic system. Including such bypass valves may increase the cost, complexity and size of the servo actuator assembly. To reduce the risk of a failure in one hydraulic system impacting on another each hydraulic system is housed separately. Maintaining two (or more) completely separate hydraulic systems in this manner may increase the size and weight of the servo actuator.
A further requirement for safety critical systems is that any structural failure of the servo actuator should be apparent from a visual inspection. Typically this means ensuring that any crack in the actuator housing will propagate such that fluid escapes to atmosphere from the valve (i.e. fluid will drip from the valve in the case of a failure). Housing each hydraulic system separately facilitates the design of servo actuators where fluid escapes to atmosphere in case of a failure.
Many of the servo actuators available, particularly for safety-critical applications have complex mechanisms designed to reduce the risk of in-service failure. Such complex mechanisms may increase manufacturing and through-life maintenance costs. It would therefore be advantageous to produce a simpler servo actuator, particularly for use in safety-critical applications.
Many servo actuators are used in applications where space is limited. Typically, reducing the size of the servo actuator leads to a reduction in the output force that can be generated by the servo actuator. Consequently, it would be advantageous to produce a servo actuator that has an increased flow rate in comparison to its size and weight. In particular, it would be advantageous to produce a servo actuator suitable for use in safety-critical applications with a reduced size and weight.